Earth Science
Chapter 9 – A View of Earth’s Past
Section 1 – Geologic Time
E.Q.: What is geologic time and the major divisions?
STANDARDS:
SES4. Students will understand how rock relationships and
fossils are used to reconstruct the Earth’s past.
e. Use geologic maps and stratigraphic relationships
to interpret major events in Earth history (e.g.,
mass extinction, major climatic change, tectonic
events).
SES5. Students will investigate the interaction of insolation
and Earth systems to produce weather and climate.
f. Relate changes in global climate to variation in
Earth/Sun relationships and to natural and
anthropogenic modification of atmospheric
composition.
SES6. Students will explain how life on Earth responds to
and shapes Earth systems.
d. Describe how fossils provide a record of shared
ancestry, evolution, and extinction that is best
explained by the mechanism of natural selection.
e. Identify the evolutionary innovations that most
profoundly shaped Earth systems: photosynthetic
prokaryotes and the atmosphere; multicellular
animals and marine environments; land plants and
terrestrial environments.
Objectives
• Summarize how scientists worked together to develop
the geologic column.
• List the major divisions of geologic time.
The Geologic Column
geologic column - an ordered arrangement of rock layers
that is based on the relative ages of the rocks and in which
the oldest rocks are at the bottom.
• Evidence of changing conditions on Earth’s surface is
recorded in the rock layers of Earth’s crust.
• The geologic time scale outlines the development of
Earth and of life on Earth.
• No single area on Earth contained a record of all
geologic time, so scientists combined observations to
create a standard geologic column.
• Rock layers in a geologic column are distinguished by
the types of rock the layers are made of and by the
kinds of fossils the layers contain.
• Fossils in the upper layers resemble modern plants and
animals.
• Many of the fossils discovered in old layers are from
species that have been extinct for millions of years.
Reading Check
Where would you find fossils of extinct animals on a
geologic column?
You would find fossils of extinct animals in older layers of
a geologic column.
Using a Geologic Column
• Scientists use geologic columns to estimate the age of
rock layers that cannot be dated radiometrically.
• To determine the layer’s age, scientists compare a
given rock layer with a similar layer in a geologic
column that contains the same fossils or that has the
same relative position.
• If the two layers match, they likely formed at about the
same time.
Divisions of Geologic Time
• The geologic history of Earth is marked by major
changes in Earth’s surface, climate, and types of
organisms.
• Geologists use these indicators to divide the geologic
time scale into smaller units.
• Rocks grouped within each unit contain similar fossils
and each unit is generally characterized by fossils of a
dominant life-form.
Eons and Eras
• The largest unit of geologic unit of time is an eon.
Geologic time is divided into four eons: the Hadean
eon, the Archean eon, the Proterozoic eon, and the
Phanerozoic eon.
• The first three eons are part of a time interval
commonly known as Precambrian Time. This 4 billion
year interval contains most of Earth’s history.
Eons and Eras
era - a unit of geologic time that includes two or more
periods
• After Precambrian time the Phanerozoic eon began.
This eon is divided into smaller units of geologic time
called eras.
• The first era of the Phanerozoic eon was the Paleozoic
Era, which lasted 292 million years.
• Paleozoic rocks contain fossils of a wide variety of
marine and terrestrial life forms.
• After the Paleozoic Era the Mesozoic Era began and
lasted about 183 million years.
• Mesozoic fossils include early forms of birds and
reptiles.
• The present era is the Cenozoic Era, which began 65
million years ago. Fossils of mammals are common in
Cenozoic rocks.
period - a unit of geologic time that is longer than an epoch
but shorter than an era
epoch - a subdivision of geologic time that is longer than
an age but shorter than a period.
• Eras are divided into shorter time units called periods.
Each period is characterized by specific fossils and is
usually named for the location in which the fossils were
first discovered.
• Where the rock record is most complete and least
deformed, a detailed fossil record may allow scientists
to divide period into shorter time units called epochs.
• Epochs may be divided into smaller units of time called
ages.
• Ages are defined by the occurrence of distinct fossils in
the fossil record.